1. Field of the Invention
The present invention relates to alternators and, more particularly, to a method of setting the idle speed of an alternator.
2. Description of the Related Art
An alternator is used to convert mechanical energy provided by an engine into electrical energy for operating electrical components and for recharging a battery. When an engine is running at idle speed, the alternator continues to provide current for operating electrical components and for recharging the battery. It may be desirable to set an appropriate engine idle speed that results in a corresponding level of output current for the alternator such that the battery may continue to supply adequate power to vehicle accessories, yet alternator wear and fuel consumption are maintained at an acceptable level. There is an engineering trade off between alternator wear and fuel consumption. The lower the engine idle speed and corresponding fuel consumption, the higher the internal alternator current that is required to produce a given level of alternator output current. The higher the internal alternator current, the higher the internal alternator temperature and the greater the rate of wear on the alternator, particularly on the brushes and windings.
It is known to set the engine idle speed to a constant level whereat a sufficient level of alternator output current is provided to the battery under all load conditions. For example, the engine idle speed may be set to a constant level that is sufficient for battery charging even when the battery is depleted and all accessories are operating under the most extreme environmental conditions. A problem with this method, however, is that the engine idle speed is set to a level that is much higher than necessary under most operating conditions. The higher engine idle speed results in unnecessary fuel consumption, engine noise and engine wear.
It is also known, in the outboard motor industry, such as used in small watercraft, to sense the instantaneous current needs of the motor and accessories, and to continually readjust the motor idle speed to the minimum speed necessary to enable the alternator to meet the instantaneous current needs. A problem with this method is that running the alternator at minimum rotational speed requires a maximum internal alternator current in order to produce the needed output current. The maximum internal alternator current results in high internal alternator temperatures and high levels of alternator wear, particularly to the alternator brushes and windings. This method may work well for watercraft, which typically have unlimited moving water available for cooling, and which typically have limited operating hours that an alternator may endure for the life of the watercraft even under stressful conditions. However, the method is less desirable for vehicles that operate in less favorable cooling conditions, and whose lives typically include many more operating hours, such as tractors. In these high operating hour applications, the high internal currents may cause the alternator to fail prematurely.
Another problem with running the alternator at a minimum rotational speed is that a relatively large alternator must be used in order to output the required current. Although the minimum idle speed may reduce the rate of fuel consumption and noise under idle conditions, at normal operating speeds the relatively large alternator puts a greater mechanical load on the engine, which results in greater fuel consumption and noise at operating speed. This too may not be a problem for a watercraft, which typically spends a large percentage of its time idling, but it is not desirable for a tractor which spends a greater percentage of its time at normal operating speed.
What is needed in the art is a method of setting engine idle speed to a variable level that does not waste fuel at idle under less-than-maximal current load conditions, but yet does not unduly burden the alternator and does not increase fuel consumption at operating speed.